U.S. Pat. No. 5,418,470 to Dagostino et al. for "Analog Multi-channel Probe System", hereby incorporated by reference, discloses a programmable analog multi-channel probe system that is suitable for embedding within an integrated circuit to be tested and for coupling test points from within that circuit to external measurement points. Programmable input buffer amplifiers, when enabled, couple the signals present at the test points on their inputs to respective routers for output. Each output of the routers is coupled to the input of an output buffer amplifier. The data at the output of those buffer amplifiers is converted to a differential signal for transmission by differential input/output amplifiers to the external measurement points. Each of the differential input/output amplifiers have selectable reference levels, including internal and external reference levels., that are used to increase their dynamic range or, in the case of the local ground signal, to improve their signal to noise ratio.
To implement analog probes for embedded testing, as shown in the U.S. Pat. No. 5,418,470 patent above, using the latest low voltage sub-micron CMOS (Complementary Metal Oxide on Silicon) circuitry and minimizing power consumption, it would be highly advantageous to have available CMOS buffer amplifiers that can both sense and drive signals having peak amplitudes that are close to the source and drain voltage rails, V.sub.SS and V.sub.DD, even when those voltages are relatively small and close together.
U.S. Pat. No. 4,730,168 to Senderowiez et al. for "CMOS Output Stage With Larger Voltage Swing and with Stabilization of the Quiescent Current", hereby incorporated by reference, describes an output stage capable of driving voltages close to the power supply rails. However, because this circuit as shown is a unity gain amplifier and the input stage is incapable of accepting input voltages that are close to the power supply rail voltages, the output voltage range is in practice to some degree limited.